1. Field of the Invention
This invention relates to shock absorbers, and more particularly to shock absorbers comprising a partition member for partitioning the interior of a cylinder into two liquid chambers while affording communication between both liquid chambers through at least one port, and a valve body biased toward said partition member for opening and closing said port.
2. Description of the Prior Art
A known partition member for partitioning the interior of the cylinder into two liquid chambers while affording communication between both liquid chambers through at least one port, for example, a piston disposed in a cylinder of a hydraulic buffer, or a valve case of a base valve disposed on the bottom of an inner cylinder in the case of a twin tube type hydraulic buffer, is configured so as to control flow of liquid in cooperation with a valve body biased toward the partition member by a spring. In particular, a known shock absorber incorporated in a suspension of an automobile has a partition member which is provided with one or a plurality of orifices in addition to the port (for example, Japanese Utility Model Public disclosure (KOKAI) No. 64536/86).
In such a shock absorber provided with an orifice, damping force is generated according to piston speed as shown in FIG. 5. That is, when the piston speed is between O and V.sub.1, liquid flows through the orifice and the damping force generated at this time by the viscous resistance of liquid increases in the form of a second-order, non linear curve as the piston speed increases. When the piston speed reaches V.sub.1 and the damping force becomes F.sub.1, the damping force is equal to the spring force acting on the valve body. When the piston speed is between V.sub.1 and V.sub.2, liquid flows, urging the valve body away from the partition member against the spring force, and the damping force increases approximately linearly. When the piston speed reaches V.sub.2 and the damping force becomes F.sub.2, the viscous resistance of liquid passing through the port is equal to the spring force. When the piston speed exceeds V.sub.2, the damping force generated by the viscous resistance of liquid flowing through the port increases in the form of a second-order, non linear curve as the piston speed increases.
The damping force of such a shock absorber is designed to have the so-called two stepped characteristics shown in FIG. 5 for the following reasons. In the region of low piston speed, up to V.sub.1, the damping force, which is relatively large and can rise rapidly according to a vehicle speed, is needed in order to restrain rolling and nose dive at the time of braking and vibration of sprung. In the region of the piston speed between V.sub.1 and V.sub.2, however, the increase in damping force as the piston speed increases is preferably restrained in order not to degrade ride comfort. When the piston speed exceeds V.sub.2 (usually the piston speed increases when travelling on a bad road), high damping force is needed in order to restrain vibration of unsprung.
When the piston speed is between O and V.sub.1 and exceeds V.sub.2, the damping force of the shock absorber is generated by utilizing the viscous resistance of the liquid. Consequently, the effect of the liquid temperature is very noticeable. That is, as shown in FIG. 5, the damping force characteristics change greatly at a low temperature T.sub.1 and at a high temperature T.sub.2 relative to the design characteristics at a temperature T.sub.0, as indicated by the three curves T.sub.0, T.sub.1 and T.sub.2. This causes the damping force to vary according to season and, in addition, immediately after use at low liquid temperature, and after a considerable period of time has elapsed and the liquid temperature has become sufficiently high, thereby greatly varying the ride characteristics of a vehicle.
To cope with the above-mentioned problems, it has been proposed, for example, as described in Japanese Utility Model Public Disclosure (KOKAI) No. 56940/80, that a bimetal member be disposed between the spring for biasing the valve body toward the piston and a member for supporting the spring to vary the spring force according to liquid temperature, thereby compensating for the change in viscosity. However such an arrangement is complicated in construction and high in cost.
As a consequence of the large variation in the damping force due to liquid temperature difference in a high speed region, the damping force in the high speed region is extremely large when the vehicle travels on a bad road at low liquid temperatures. Therefore ride comfort is degraded. Further, the strength of the mounting section of the shock absorber on the car body will be greatly damaged by the excessive damping force. Also, at high liquid temperatures or when travelling on the bad road for a long time, the damping force in the high speed region becomes too small, which causes the wheel stroke to become too large and the bound and rebound stoppers to be frequently hit. As a result, not only ride comfort is degraded, but also the strength of the mounting section on the car body is greatly damaged.